Income Taxes - Summary of Deferred tax assets and liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2018	Dec. 31, 2019
Gross deferred tax assets
Net operating loss carryforwards	$ 174,369		$ 172,745
Tax credit carryforwards	44,936		42,642
Deferred research and development expenses	63,215		70,042
Stock-based compensation	13,196		12,651
Fixed assets	1,303		1,759
Accrued expenses and other	325		328
Gross deferred tax assets	297,344		300,167
Gross deferred tax liabilities
IPR&D intangibles	7,802		12,748
Total deferred tax assets and liabilities	289,542		287,419
Valuation Allowance	(291,382)		(290,426)
Net deferred tax liability	1,840		$ 3,007
Principal components of the deferred tax assets and liabilities
Income Tax Benefit	$ (1,167)	$ (765)